Metal recovery process



METAL RECOVERY PROCESS Louis B. Werner and Grville F. Hill, Richland, Wash, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application June 11, 1945, Serial No. 598,908

11 Claims. (Cl. 2314.5)

The present invention is directed to a method for the recovery of plutonium from solid compositions containing the same. More particularly, it is related to a novel process for the recovery of plutonium in concentrated form from solid compositions containing columbic oxide by the formation of volatile fluorides of certain fission products and columbium followed by the formation of a volatile fluoride of plutonium and the subsequent recovery thereof from the relatively nonvolatile fluorides of other impurities associated therewith.

It is known that plutonium can be produced in small quantities by the bombardment of uranium metal with slow or thermal neutrons. The designation plutonium or element 94 as used throughout the present description refers to the transuranic element having an atomic number of 94. The expression 94 means the isotope of element 94 having an atomic Weight or mass of 239. Similarly, the terms element 93 or Np refer to the new element known as neptunium having an atomic number of 93.

Uranium metal is composed of three isotopes, namely, U U and U the latter being present in excess of 99 percent of the whole. When U is subjected to the action of slow or thermal neutrons, a third isotope, U is produced having a half-life of 23 minutes and undergoes beta decay to Np which decays further by beta radiation with a half-life of 2.3 days to yield plutonium. In addition to the formation of the transuranic elements, neptunium and plutonium, there are simultaneously produced other elements of lower atomic weight known as fission fragments. These fission fragments are composed of two distinct element groups, i. e., a light and heavy element group. The light group contains elements having atomic numbers of between about 35 and 46 while the heavy group is composed of elements of atomic numbers varying between about 51 and 60. The elements of these groups as originally produced are in the form of highly unstable isotopes, and by beta radiation quickly transform themselves into isotopes of these various elements having longer half-lives. The resulting materials are commonly known as fission products.

The various radioactive fission products have half-lives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be substantially eliminated by ageing the material for a reasonable period before handling. Those with very long half-lives do not have sufficiently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radiations which cannot be eliminated by ageing for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce, and Pr of the heavy group.

It may readily be seen that plutonium produced as generally set forth above is contaminated with considerable quantities of uranium and fission products. In fact, the plutonium usually constitutes only a very minor poratom tion of the irradiated mass, i. e., less than one percent thereof. In view of such a low concentration of plutonium in the irradiated metal, it becomes apparent that the procedure employed to recover that element must be highly efiicient in order to be at all practicable.

There have been devised a number of procedures for the removal and concentration of plutonium from extremely dilute solutions thereof. In general, such methods in volve the formation of various insoluble compounds in said dilute solutions capable of carrying plutonium in the reduced state. The carrier precipitate and plutonium thus obtained are then dissolved and the plutonium oxidized to PuO in which state of oxidation it is soluble in the presence of said carrier. Under these conditions, the plutonium remains in solution and the fission products are removed when the carrier is added. Thereafter, the dissolved plutonium is reduced to a valence state in which it is carriable by the aforesaid carrier and removed from solution in the form of a carrier precipitate which may again be dissolved and the plutonium purified further if considered necessary or desirable by repeating the above cycle. This procedure, however, is obviously cumbersome and time consuming since it requires a number of such steps in order to effect a substantial removal of fission products and other impurities. Moreover, certain fission products, notably columbium and zirconium, are extremely difficult to remove from plutonium-containing solutions of the aforesaid type by the use of carriers in accordance with the method generally outlined above.

Of the various materials suitable for use as carriers of plutonium in the reduced state, in accordance'with the procedure generally set forth above, it has previously been found that columbic oxide is admirably suited for this purpose. However, it has also been observed that columbic oxide, like the other compounds used as carriers in such a process, carries a considerable quantity of impurities, chiefly fission products, together with the plutonium, and hence the procedure of carrying plutonium in its reduced state, dissolving the precipitate thus formed, oxidizing the plutonium to a noncarriable state, removing the fission products by precipitating the columbic oxide carrier, etc., had to be repeated several times before the plutonium could be obtained in a substantially pure condition.

It is an object of the present invention to provide a single step process for obtaining plutonium values in high concentration from acid solutions of neutron bombarded uranium.

It is a further object of our invention to provide a method that will effectively separate plutonium from the fission products and other impurities normally associated therewith.

Additional objects of our invention will be apparent as the present description proceeds.

It has now been discovered that plutonium can be obtained in a highly concentrated and purified state in the form of its volatile fluoride from columbic oxide carrier precipitates by means of a procedure involving selective hydrofluorination and fluorination processes.

The recovery and concentration of plutonium in accordance with the process of the present invention is based upon the fact that columbic oxide is converted to volatile columbium pentafiuoride when the former is sub jected to the action of hydrogen fluoride whereas plutonium in the tetravalent state under the same conditions is converted to plutonium tetrafluoride, a substantially nonvolatile compound. After the columbium has been removed as the fluoride, the residue is contacted with elemental fiuorine to convert the plutonium tetrafluoride contained therein to a higher volatile fluoride, probably the penta or hexafiuoride, or a mixture thereof, and hence may be readily separated as such from the residual impurities whose fluorides are substantially nonvolatile.

In accordance with a preferred embodiment of our invention, neutron bombarded uranium-is first dissolved;

in a suitable acid suchas, for example, nitricacid. Since the plutonium presentin the resulting solutionexists in different valence states, it is preferable to subject said solution to the action of a reducing agent suchas, for example, sodium nitrite, inorder to convertcompletely the dissolved plutonium to a carriable state (valence, state not above 4). To the solution thus treated is then added a preformed columbic oxide precipitate either in the anhydrous or hydrated form, thereby causing the substantially complete precipitationbfplutonium. Since the dissolved hexavalent uranium is unafiectedby the aforesaid reduction step, it is not removed ,from solution by the columbic oxide. carrier. However a. substantial amount of radioactive fission products,particularly, columbium andzirconium are brought v down, Additional fission products present atthis stages thanare, capable of forming volatile fluorides are antimony, ruthenium, and element43. The columbic oxideprecipitate thusobt'ained is next dried and thereafter placed ma lsuitable fluorina tion reactor. The dried precipitateiis.then subjected, at elevated temperatures, for example ofgtheorder of about.

500 C., to the action ofhydrogen; fluoride, preferably in the anhydrous condition, whereupon the ,colum'biumpfes: out together with such elemeirtsJasjantimony, ruthenium, and element 43 areconverted-to their volatile fluorides whichare readily separated from theremainder of the solid mass. iodine are also volatilized at this point. The residue which consists essentially ofplutonium tetrafluoride and substantially nonvolatilefission product fluoridesincluding the fluorides of rubidium, strontium, barium, yttrium, lanthanum, cesium, and cerium are thencontacted with fluorine, preferably in the anhydrous state, at temperatures of about 315 'C. or above, and preferablybetween about 425 and 500 C to convertthe lower plutonium fluoride. to a higher volatile fluOride, (probablya fluoridefof plu-. tonium .in valencestate. above 4) which is collected. attemperatures below 315" C, in a suitablereceiver. molybdenum and neptunium present may be separated from the plutoniumacontaining ,mass by treatmentwith fluorine at temperatures betweenaboutZSO and 300 C. prior to formation of the higher fluorideof plutonium;

Any columbium which is not removed on reaction of,

the original carrier precipitate with hydrogen ,fluoride under the above stated conditionswill,ordinarily,v be

removed along with theplutonium hexaflucride when the aforesaid residue is subjected to the action of elemental. fluorine, after which the columbium may.be separated from plutonium by fractional distillation owing vto,- a difference of approximately 70 C. in the boiling points of their fluorides.

Our invention may be. further illustrated by the follow ing specific example:

Example To 25 ml. of a nitric acid solution of neutron irradiated uranium, which has previouslybeen treated with sodium nitrite to reduce the plutonium present therein to a carriable state, is added 1 g. of a preformed columbic oxide precipitate. The resulting mixture is agitated. for ten minutes, after which it is allowed to settle and the columbic oxide carrier precipitate separated by centrifugation. The solid residue thus obtained, and which contains approximately 50 mg. of plutonium together with fission products, is next placed in a drying oven at a Other fission products such as bromine. and

The

temperature of about 100 C. andallowed to remain until substantiallyanhydrous.- Thereafterthe-dried columbic oxidexcarrier precipitate is .placednin a nickeltube fiuorination reactor, of conventional; design after which anhydrous hydrogen fluoride is ,introduced ,tat; a

rate of approximately 7 0.5 liter, per hourfor ;a ;period,;of

1, the columbium is converted to columbic pentafluoride and is completely volatilized together with bromine and iodine, and the fluorides of zirconium, ruthenium, and antimony. Under these conditions the plutonium present is converted to plutonium tetrafluoride which is nonvolatile and is left as a residue in the reaction. chamber together with such fission products as barium, strontium, neptunium, molybdenum, yttrium, lanthanum, cesium, and cerium.

Anhydrous fluorine is next introduced at a rate of about.

0.3 liter per hour at a temperature of between 250300 C. whereupon any neptunium and molybdenum which might be present are converted to their volatile fluorides and thusremoved. Thetemperature is then increased to about 500 C. under which conditions the plutonium tetrafluoride is transformed to a volatile fluoride and is collected in a suitable receiving vessel at a temperature below 315 C. The remaining fission products, most of which are in the form of their fluorides, are left as a residue in the-reaction chamber. The plutoniumrecovered in this manner representsapproximately 90-95 percent of that originally present in the columbic oxide carrier precipitate.

It will be apparent to those skilled in the art that the method of recovering plutonium as generally set forth.

above provides a simple and practical procedure for the procurement of high concentrations of plutoniumin a single step. Also, it will be further apparent that by the utilization of such a procedure, a relatively high degree of purification ofplutonium with respect to fission products and other impurities is obtainable.

While this inventionhas been illustrated by certain restricted applications thereof, it is not desired to be specifically limited thereto, since it is manifest to those skilled in theart to. which the present invention is directed that it is susceptible tonumerous alterations and modifications without departing-from the scopethereof. For example, anhydrous' liquid hydrogen fluoride instead of.

gaseous hydrogen fluoride may be employed in the formation of the desired fluorides in our process. In general, it mayv be said. that any process for the separation of plutonium from columbic oxide carrier precipitates and the impurities associfitedtherewith by formation of volatile fluorides of said columbic oxide and impurities followed bvformation ofayolatile fluoride of plutonium andsubsequent recovery thereof, is to be construed as lying within the scope of our invention.

What isclaimed:

1. The process of recovering a volatile fluoride of plutoniurnin concentrated 1 and purified form from a solid composition containingcolumbic oxide, plutonium values, and fission product values which comprises contactingv said compositionwithhydrogen fluoride, heating said-com:

position andhydrogen fluoride to a temperature of about uct fluorides, and recovering saidvolatile fluoride ofplutonium.

2. The process of recoveringa volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and fission product values which comprises contactingnsaid composition with anhydrous hydrogen fluoride, heating said composition and hydrogen fluoridetoa temperature of about 500- C. to convert the plutonium values to nonvolatile plutoni-um tetrafluorideandto volatilize the co: lumbium values as ,columhiumpentafluoride together. with. two hours at 500? C. At theend of thisperiod all; of volatile fission;- produqt. fluorides,,contacting the, residual nonvolatile fission product fluorides and plutonium tetrafiuoride with anhydrous fluorine, heating this mixture of plutonium tetrafiuoride, fission product fluorides, and fluorine to a temperature of between 315 -500 C. to form a volatile fluoride of plutonium leaving as a residue the substantially nonvolatile fission product fluorides, and recovering said volatile plutonium fluoride.

3. The process of recovering a volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and fission product values which comprises contacting said composition with hydrogen fluoride, heating said composition and hydrogen fluoride to a temperature of about 500 C. to convert the plutonium values to nonvolatile plutonium tetrafiuoride and to volatilize the columbium values as columbium pentafluoride together with volatile fission product fluorides, contacting the residual nonvolatile fission product fluorides and plutonium tetrafluorides with fluorine, heating this mixture of plutonium tetrafiuoride, fission product fluorides, and fluorine to a temperature of between 425 500" C. to form a volatile fluoride of plutonium leaving as a residue the substantially nonvolatile fission product fluorides, and recovering said volatile plutonium fluoride.

4. The process of recovering a volatile fluoride of plutonium in concentrated and purified form which comprises removing plutonium values from an acid solution containing uranium values in the hexavalent state, fission product values, and plutonium values in the tetravalent state by adding thereto columbic oxide, removing the resultant precipitate which contains plutonium values and fission product values, drying said precipitate to a substantially anhydrous condition and thereafter subjecting said precipitate to the action of hydrogen fluoride, heating the mixture of said precipitate and hydrogen fluoride to a temperature of about 500 C. to convert the plutonium values to nonvolatile plutonium tetrafiuoride and to volatilize columbium pentafluoride and volatile fission product fluorides, contacting the residual nonvolatile fission product fluorides and plutonium tetrafiuoride with fluorine, heating this mixture of plutonium tetrafiuoride, fission product fluorides, and fluorine to a temperature of between 315 and 500 C. to form a volatile fluoride of plutonium leaving as a residue the substantially nonvolatile fission product fluorides, and recovering said volatile plutonium fluoride.

5. The process of recover-ing a volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and fission product values which comprises hydrofluorinating said composition to convert the plutonium values to nonvolatile plutonium tetrafiuoride, volatilizing columbium values as columbium pentafluoride together with volatile fission product fluorides by subjecting the resultant hydrofluorinated mass to a temperature of about 500 C., fluorinating the nonvolatile residue thus obtained which comprises essentially plutonium tetrafiuoride and nonvolatile fission product fluorides to convert said plutonium tetrafiuoride to a volatile fluoride of plutonium, separating the latter from the remainder of the nonvolatile fluorinated residue by heating said residue to the volatilization temperature of said volatile plutonium fluoride leaving a residue comprising essentially nonvolatile fission product fluorides, and thereafter recovering said volatile plutonium fluoride.

6. The process of recovering a volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and fission product values which comprises subjecting said composition to the action of anhydrous hydrogen fluoride at a temperature of about 500 C. to convert the plutonium values to nonvolatile plutonium tetrafiuoride and to volatilize columbium values as columbium pentafluoride together with volatile fission product fluorides, subjecting the nonvolatile residue thus obtained to the action of anhydrous fluorine at a temperature of above about 315 C. to form a volatile fluoride of plutonium leaving as a residue the nonvolatile fission product fluorides, and recovering said volatile plutonium fluoride.

7. The process of recovering a volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and radioactive fission products, which comprises subjecting said composition to the action of anhydrous hydrogen fluoride at a temperature of about 500 C. to convert the plutonium values to nonvolatile plutonium tetrafluoride and to volatilize columbium values as columbium pentafluoride together with the Volatile radioactive fission product fluorides, fluorinating the residual nonvolatile radioactive fission products and plutonium tetrafiuoride at a temperature of between about 425 and 500 C. to form a volatile fluoride of plutonium leaving as a residue the substantially nonvolatile radioactive fission product fluorides, and recovering said volatile plutonium fluoride.

8. The process of recovering a volatile fluoride of plutonium in concentrated and purified form from a solid composition containing columbic oxide, plutonium values and fission product values which comprises contacting said composition with hydrogen fluoride to convert said plutonium values to plutonium tetrafiuoride, and said fission product values and columbium values to fission product fluorides and columbium pentafluoride re spectively, heating the resulting hydrofluorinated mass to a temperature at which plutonium tetrafiuoride is nonvolatile and that is the volatilization temperature for the columbium pentafluoride and volatile fission product fluorides thus produced, fluorinating the resultant non-volatile residue, separating the volatile fluoride of plutonium thus formed from the nonvolatile remainder of the fluorinated composition by heating said composition to the volatilization temperature of said plutonium fluoride leaving a residue comprising essentially nonvolatile fission product fluorides, and recovering said volatile plutonium fluoride.

9. In a process for the recovery of plutonium tetrafiuoride in concentrated and purified form from a solid composition containing columbic oxide, plutonium values, and fission product values, the steps which comprise contacting said composition with anhydrous hydrogen fluoride to form plutonium tetrafiuoride, columbium pentafluoride, and fluorides of said fission products, separating said plutonium tetrafiuoride from said columbium pentafluoride and volatile fission product fluorides by heating the hydrofluorinated mass to the volatilization temperature of said columbium pentafluoride and volatile fission product fluorides, leaving a nonvolatile residue comprising plutonium tetrafiuoride and nonvolatile fission product fluorides.

10. In a process for the recovery of plutonium tetrafluoride in concentrated and purified form from a solid composition containing columbic oxide and plutonium values, the steps which comprise contacting said composition with hydrogen fluoride and heating the resulting hydrofluorinated mass to the volatilization temperature of the columbium pentafluoride thus produced, leaving as a nonvolatile residue a composition comprising plutonium tetrafiuoride.

11. In a process for the recovery of plutonium tetrafluoride in concentrated and purified form, the steps which comprise contacting an aqueous medium containing plutonium in a valence state not greater than 4 with columbic oxide, separating the resulting solid composition comprising plutonium values and columbic oxide, thereafter contacting said composition with hydrogen fluoride, and heating the resulting hydrofluorinated mass to the volatilization temperature of the columbium pentafluoride thus produced leaving as a nonvolatile residue a composition comprising plutonium tetrafiuoride.

No references cited. 

1. THE PROCESS OF RECOVERING A VOLATILE FLUORIDE OF PLUTONIUM IN CONCENTRATED AND PURIFIED FORM FROM A SOLID COMPOSITION CONTAINING COLUMBIC OXIDE, PLUTONIUM VALUES, AND FISSION PRODUCT VALUES WHICH COMPRISES CONTACTING POSITION AND HYDROGEN FLUORIDE TO A TEMPERATURE OF ABOUT 500* C. TO CONVERT THE PLUTONIUM VALUES TO NONVOLATILE PLUTONIUM TETRAFLUORIDE AND TO VOLATILIZE THE COLUMBIUM VALUES AS COLUMBIUM PENTAFLUORIDE TOGETHER WITH VOLATILE FISSION PRODUCT FLUORIDES, CONTACTING THE RESIDUAL NONVOLATILE FISSION PRODUCT FLUORIDES AND PLUTONIUM TETRAFLUORIDE WITH FLUORINE, HEATING THIS MIXTURE OF PLUTONIUM TETRAFLUORIDE, FISSION PRODUCT FLUORIDES, AND FLUORINE TO A TEMPERATURE OF BETWEEN 315*-500*C. TO FORM A VOLATILE FLUORIDE OF PLUTONIUM WHICH IS IMMEDIATELY VOLATILIZED LEAVING AS A RESIDUE THE SUBSTANTIALLY NONVOLATILE FISSION PRODUCT FLUORIDES, AND RECOVERING SAID VOLATILE FLUORIDE OF PLUTONIUM. 